1. Field of the Invention
The present invention relates to a liquid container for storing a liquid such as ink, used to supply the ink to a print head of an ink jet printing apparatus. More specifically, the present invention relates to a liquid container for storing ink containing pigment as a colorant.
2. Description of the Related Art
The ink jet printing apparatus forms an image on a print medium by ejecting ink from a plurality of nozzles in a print head onto the print medium. In such ink jet printing apparatus, there are two types of ink tank for installed supplying ink to the print head: an ink tank of relatively small capacity is adapted to be mounted on a carriage together with the print head, and an ink tank of relatively large capacity is adapted not to be mounted on the carriage but to supply ink through a supply member to the print head. The ink tank of relatively large capacity not mounted on the carriage is often removably connected to an end of an ink supply system that supplies ink to the print head. The conventional removable ink tank is known either to use in the ink tank with a member for generating a capillary force, such as a sponge, for holding ink or to directly store ink in a flexible bag or in a stiff case. In a wide format printer with a large ink consumption volume per print medium or in a network printer with a high operating efficiency, in particular, a large volume of ink is required. Hence, considering a reduction in an ink tank replacement frequency and an ink storage efficiency, an ink tank of the type that directly stores ink in the tank without using a member such as sponge has been desired.
In such an ink tank of directly storing ink, a dye ink capable of keeping an ink concentration uniformity at all times has been used. However, printed products using the dye ink have poor light, gas and water resistances. Therefore, the dye ink is not suited for print materials for outdoor display and for long-term storage. A pigment ink that uses pigment as a colorant, on the other hand, has excellent light, gas and water resistances, compared with the dye ink. However, since pigments are dispersed, not dissolved, in a solvent, they will precipitate in a static ink tank.
The small-capacity ink tank integrally formed with the print head and mounted on the carriage is vibrated by a scanning action of the carriage, which agitates the ink contained therein. Hence, the print head is supplied with the ink that has a pigment dispersed to produce a relatively uniform concentration. On the other hand, the large-capacity ink tank that is not mounted on the carriage is fixed stationary in a predetermined position from which it supplies ink, so that a phenomenon of the pigment precipitating in the tank cannot be ignored depending on a frequency of ink supply, an interval between ink supply operations and the number of print mediums to be printed.
For example, in an ink tank that is left standing for many hours, a pigment settles, there is a gradient of pigment particle concentration from a layer of excessively dark color at a tank bottom to a layer of excessively light color near a liquid surface in the ink tank, resulting in a significant ink concentration difference between the tank bottom and the liquid surface. If the ink tank has a construction in which ink is extracted from the bottom of the ink tank, an excessively high concentration ink is extracted first. Suppose a great deal of printing is performed in this condition. A comparison between a printed result obtained immediately after a start of the printing operation and a printed result obtained immediately before an end of the printing operation may indicate a recognizable color difference. This phenomenon is particularly remarkable with a color printing that forms an image with varying color densities.
To solve this problem, a tubular member having a plurality of holes on the circumference thereof is extended from an ink supply port of the ink tank into the interior of the ink tank so that ink is drawn out not only from portions near the ink supply port but also from many vertically spaced positions in the ink tank. The ink drawn out from these holes is temporarily stored in an ink holding portion, from which it is then supplied to the print head, thereby reducing concentration variations in the supplied ink (e.g., Japanese Patent Application Laid-open Nos. 2001-270131 and 2001-293880).
Such a conventional ink tank also has the following problem.
First, ink from a variety of layers of different concentrations in the ink tank flows into the tubular member through a plurality of holes and mixes in the tubular member and the ink holding portion to become uniform in concentration. However, even in the tubular member and the ink holding portion, pigment precipitation occurs and therefore ink concentration variations may result. To solve this problem, it may be conceived to draw out ink from the tubular member as by a pump and discard it outside. But this method wastes ink and is not an effective method.
Further, a low-concentration ink in the upper part of the ink tank, because it contains a smaller amount of pigment, is lighter than a high-concentration ink at the bottom of the tank. As described above, ink flows into the tubular member evenly from various layers in the ink tank. In the tubular member, however, a heavy ink that has flowed in through the holes into a lower part of the tubular member wall occupies a vertically lower space in the tubular member while a light ink that has flowed in through the holes in an upper part of the tubular member wall occupies a vertically higher space in the tubular member. It is therefore difficult for a vertical convection between the heavy ink and the light ink to be set in motion, leaving a possibility of the heavy and light inks not mixing uniformly as it is supplied to the print head.